Hosts are the skulls that occasionally raise themselves up to shoot at you. When they raise themselves, that is the only time you can damage them. I can't seem to find any sort of pattern and end up spending 10 minutes running around hoping this thing reveals itself. Is there anything I can do to make them raise up?

You'll occasionally encounter red Hosts, who aren't invulnerable when they're down. I think they do more damage to make up for it, but I'm not sure.
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TacroyApr 1 '12 at 14:35

@Tacroy I think they just shoot more bullets (5 at a time).
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lunboksApr 1 '12 at 15:20

I can run around a Host without shooting them for minutes and they will do nothing.
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user9983Apr 1 '12 at 17:41

@OrigamiRobot that's really weird, I just played a round to see whether an update broke something, but for me hosts start shooting again after I've moved about half a room's width away from them for a few second and return to them. Has this always been like that? Also note that if they look like this (usually from a bomb while defending) they take some time to recover
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ZommuterApr 1 '12 at 18:40

2

1 is wrong. Standing still for long enough (in range) is perhaps the best way to deal with hosts.
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badp♦Jul 1 '12 at 11:30

From my experience, it is not too difficult to "bait" hosts into shooting at you. Try standing directly above/below/to the side of the host (not necessarily close to it, just so that it can see you in a direct line in one of the four cardinal directions) and making small movements in various directions.

If the host is really ignoring you I also try shooting away from it. That usually did it for me (unless it's completely psychosomatic, and I only felt like it helped).

I've found that they are vulnerable to spikes on the floor, even when down. Shooting at them while down pushes them around a bit; it is thus possible to maneuver them onto nearby spikes and eliminate them from a distance.

While it's situational at best, I'd like to add that, even while down, hosts receive damage from The Virus, and they deal no damage via touch in return, so if you happen to have that item, you can shoot to keep them down after their initial attack, then kill them that way.

Based on their wiki page they are also vulnerable to Dead Bird while down.

For lone hosts, the best thing you can do is stop shooting and stand still in range (a little more than half the screen's width). Eventually the host will attack, allowing you to attack first and dodging his own bullets later.

When you have a group of three hosts in a line, wait for one to attack and then unleash on all three. This will attack all hosts that open together and prevent closed hosts from opening themselves for a while. Repeat until they're all dead.